New areas of investigation of synaptic structure are underway. A method for staining freeze-substituted issue has been developed which requires no further stain afer the sections are cut, so the stain extends evenly through the section. Therefore the three dimensional structure of the cytoskeleton and related fine filaments in synapses can be determined in continuous serial sections. How neurofilaments end in synaptic terminals has been determined; this is important because neurofilament lengths are thought to be regulated by C-activated proteases at their terminations. Application of the freeze-fracture techniques has shown that the pattern of active zone stucture at synapses on fast muscle fibers differs from that on slow muscle fibers; these structural differences provide a basis for understanding why termials on fast fibers release more transmitter quanta than those on slow fibers. Growing nerve terminals in the brain have been reconstructed from serial sectioned freeze-substituted preparations. These new preparative methods have revealed an internal system of membranes which are thought to be the source of the new membrane added to the surface of the growth cone during its growth. These membranes are highly labile and are destroyed by conventional fixatives. Current evidence indicates that they participate in recycling of membranes needed for extension of the growth cone.